(a) Technical Field
The present invention relates to a laser apparatus using anisotropic crystals. More particularly, the present invention relates to a laser apparatus using anisotropic crystals, which can achieve a femtosecond laser system pumped optically by laser diodes by improving the quality of a pulse beam while increasing a laser output power and also reducing the width of a pulse.
(b) Background Art
An ultrashort laser light source such as a femtosecond laser pulse generates an ultrashort pulse of high peak power. Also, since the average output power of the pulses is relatively high, the ultrashort laser light source is being widely used in basic science fields such as ultrahigh speed spectrochemistry, high energy physics, and XUV-wave generation as well as in various fields such as ultrafine laser processing and micro surgery.
Generally, the ultrashort laser pulse has excellent characteristics such as high peak power, wide spectrum bandwidth etc as well as the short pulse time width.
Since the ultrashort laser pulse can be applied to micro- or nano-processing of electronic components or optical components such as solar cells, optical memories, semiconductors, and flat panel displays that require a high level of precision, demands for industrial ultrashort pulse laser systems are being increasing.
In order to satisfy the above demands, conditions for applying an ultrashort laser pulse to ultrafine laser processing will be first described as follows.
First, since the laser pulse time width is considerably shorter than the electron-phonon relaxation time of a target material, thermal energy should not be transferred to the surroundings of a part to be processed during the processing.
(Non-Thermal Processing)
This is called “cold ablation”. For example, the electron-phonon relaxation times of aluminum, iron, and copper are 4.27 picoseconds (ps), 3.5 ps, and 57.5 ps, respectively.
In other words, when the aluminum is processed by the ultrafine laser processing, for the cold ablation, it is desirable to apply a laser pulse with a pulse time width of picosecond or less.
Accordingly, a femtosecond laser is most suitable for the ultrafine laser processing of the cold ablation.
Since the ultrashort laser pulse of the femtosecond range minimize the thermal diffusion in the process region and does not give damage to the surroundings due to residual heat, even very hard materials that are difficult to perform mechanical processing can be processed. Also, since the pulse time width is short and the pulse energy and the peak power are high, various kinds of nanometer scale superfine structure processing are possible even for the transparent materials such as glass and polymer by way of the nonlinear optical effect that is called multi-photon absorption.
Second, the target materials to be processed have ablation thresholds of about several J/cm2 or more. Considering the size of the laser beam that is focused on a target part for the ablation processing, pulse energy of about 10 μJ or more is required.
Several cases of material process application require pulse energy of hundreds of μJ.
One of representative lasers having such excellent characteristics is a titanium sapphire laser.
The titanium sapphire lasers that are commercially available so far provide a pulse time width of about several to hundreds femtoseconds and a pulse energy of several mJ or several J.
However, since an expensive high-output pulse green laser such as Nd:YVO4 laser has to be used as a pumping light source, it is difficult to obtain a pulse repetition rate of tens of kHz or more.
Also, the titanium sapphire laser is large in system scale and expensive, and is hard to stably maintain the pulse output, it is not easy to utilize at a production site.
On the other hand, as a diode-pumped solid-state (DPSS) laser uses a small size of light source such as a laser diode as a pumping light source, and configures the femtosecond laser with solid laser materials, the optical pumping structure is simplified. Accordingly, the size of a laser head size becomes smaller, and the laser diode of a wavelength widely used in various fields for the commercial purpose is cheap compared to its output. Also, since the price of the femtosecond laser can be reduced, there is a cost-saving effect.
Also, the solid laser has a short optical pumping distance and thus the stable laser operation is possible. Thus, the solid laser is suitable for application to industrial lasers.
Recently, as a laser diode array and a laser diode bar which are very small but very efficient and are of stable and high power due to advancement of the semiconductor and electronic engineering are being developed, the solid laser system using diode pumping is being rapidly developed.
In order to realize the femtosecond laser system where the optical pumping is conducted using the laser diode, it is essential to select laser materials or laser crystals complying with requirements and then design and manufacture an optical pumping module for the effective optical pumping.
As a main laser material for the diode pumping, crystals doped with rare-earth ions such as neodymium (Nd) and ytterbium (Yb) which can be pumped with a laser diode of 808 nm and 980 nm range are being widely used.
At the initial stage of the development of the high output lasers, laser crystals doped with Nd have been preferred because they have a 4-level structure and various absorption lines. However, in recent years, crystals doped with Yb, which have a simpler energy level, are being widely used by showing more excellent thermal and optical characteristics.
Widely-used laser materials among materials doped with Yb are divided into non-crystal materials and crystal materials according to the form of the mother material. The crystal materials are divided into isotropic crystals and anisotropic crystals such as uniaxial crystals and biaxial crystals.
Examples of non-crystal materials include Yb:glass, and examples of isotropic crystals include Yb:YAG, Yb:ScO, Yb:YO, Yb:LuO, Yb:LuScO, and Yb:CaF. Examples of uniaxial crystals include Yb:CALGO, Yb:YVO4, Yb:NGW, Yb:NYW, Yb:LuVO, Yb:LSB, Yb:S-FAP, and Yb:C-FAP, and examples of biaxial crystals include Yb:KYW, Yb:KGW, Yb:KLuW, and Yb:YCOB.
Among these, the monoclinic double tungstates such as Yb:KYW, Yb:KGW and Yb:KLuW doped with Yb can generate a high pulse energy and a high average output due to the excellent laser characteristics and thermal-mechanical characteristics, and thus has sufficient conditions to be used as a femtosecond laser material.
Particularly, Yb: KYW or Yb:KGW laser crystal has similar characteristics and has large emission and absorption cross-sections and a large emission bandwidth among the crystals doped with ytterbium, and thus can produce pulses of 200 femtoseconds or less. Also, since having sufficient thermal conductivity, Yb:KYW or Yb:KGW laser crystal has an advantage of producing a femtosecond laser of a high average output.
Yb:KYW or Yb:KGW is not symmetrical with respect to the rotation axis of the crystal regarding electrical, optical and mechanical characteristics, and is an anisotropic laser crystal that shows different characteristics according a specific axial direction.
Particularly, regarding the optical characteristics, since the refractive index, absorption degree and emittance with respect to the wavelength, thermal conductivity, and thermal expansion rate are different according to the axial direction of the laser crystal, when these anisotropic laser crystals are used as the laser materials, it is important to appropriately select the axial direction of the crystal.
There are additional requirements in order to apply the femtosecond laser light source to the industrial site involving ultrafine laser processing.
For example, if the pulse repetition rate of the laser is low, the laser processing takes much time, causing reduction of the productivity at production sites.
However, while the pulse repetition rate of the laser is better off being higher, there is a limitation in increasing the pulse repetition rate.
If the pulse repetition rate is too high such that the next laser pulse comes before plasma generated by the femtosecond laser pulse dissipates, the next laser pulse may be affected by the plasma existing around the target part, allowing the travelling direction of the beam or the pulse time width to change.
This is called plasma shielding.
In order to prevent the plasma shielding effect, the next laser pulse must be applied after the plasma relaxation time passes.
In other words, the pulse time interval between laser pulses needs to be longer than the plasma relaxation time. Though the plasma relaxation time varies according to materials to be processed, based on the laser pulse repetition rate, its repetition rate is about 1 MHz.
Accordingly, in order to maintain a high productivity in the production site, a femtosecond laser having a pulse repetition rate of hundreds of kHz range is required.
Also, in order to mount and operate the laser light source in the laser process system, high operational stability in which the laser operation status does not change over a long period of time as well as a compact size and a low price are required.
When a femtosecond pulse occurs for the first time at mode locking in a femtosecond oscillator, its pulse energy is very low at a level of nanojoule (nJ), and thus it is not suitable for application to the laser processing.
In order to increase the femtosecond pulse energy, the chirped pulse amplification (CPA) technology is used.
For example, by using a pulse stretcher, a pulse coming from the femtosecond oscillator is expanded in terms of time and then applied to the amplifier to amplify the pulse energy.
Then, the amplified pulse is allowed to pass through a pulse compressor to return the time width of the pulse to the original femtosecond range.
In this case, the pulses coming from the femtosecond oscillator serve as seeding pulses applied to the amplifier.
The temporal expansion of the pulse due to a path difference according to the wavelength in the pulse stretcher is called chirping, and the amplifying technology of the pulse energy through this process is called a chirped pulse amplification technology.
When using this technology, the nonlinear deformation that occurs in the temporal and spatial distribution of the laser pulse due to the self-focusing effect can be inhibited by maintaining the peak power in the resonator of the pulse amplifier at the necessary level, and also the physical damage that can be applied to the optical components constituting the system can also be prevented.
In other words, damage of the system due to a high energy of laser pulse can be prevented, and the pulse amplifier can be effectively operated for the enhancement of the pulse energy.
Recently, based on the chirped pulse amplification technology, as the high pulse energy can be obtained in a MOPA system which is a combination of a femtosecond master oscillator (MO) that is directly pumped by a diode light source and a power amplifier (PA) that is directly pumped by a diode light source, a significant advancement is being made in the development of the femtosecond laser system that has a high peak power and a high average output.
However, since the Yb-doped laser material has a 2-level energy structure or a near 3-level energy structure, the laser material has a disadvantage in that the light emitting at an optical pumping wavelength of 981 nm is absorbed back into the laser material.
In order to overcome the above limitation, a high power of high-brightness laser diode light source is focused on a very small spot in the laser crystal.
During this process, the pumping light source that is not deformed into the laser beam is transferred to the surroundings of the spot of the laser crystal in a form of thermal energy and is also transferred to the mount to which the laser crystal is coupled.
If a large amount of thermal energy is accumulated during this process, the quality of the beam is degraded due to distortion of the amplified laser beam, and the laser average output and the pulse energy are also limited.
Also, if the thermal energy accumulated in the laser crystal becomes higher than a damage threshold, physical damage such as cracks in the laser crystal or breakage of the laser crystal may occur, causing interruption of laser oscillation.
Hereinafter, the previous studies of generating or amplifying a femtosecond pulse using the plurality of Yb:KYW or Yb:KGW laser crystals will be described.
For example, U.S. Pat. No. 7,508,847 B2 discloses a method of increasing the frequency in which the laser beam passes through the gain material in the resonator by optical-pumping two anisotropic materials such as Yb:KGW.
However, the cited patent is focusing on increasing the length of the laser resonator in order to reduce the unstable phenomenon of the pulse power called a triggered mode. Also, the cited patent is merely proposing two gain material pumping configuration as one of various types of long resonators, but is not proposing any experimental examples or results.
Also, U.S. Pat. No. 6,760,356 B2 discloses a concept of amplifying a femtosecond pulse using two Yb:YAG that are isotropic crystals.
However, the greatest difference between the two cited patents and the present invention is that the two cited patents focus on amplifying the output power using only two laser materials but the present invention does not focus only on amplifying the output using anisotropic laser crystals that shows different characteristics depending on the axial direction but also on increasing the spectrum width of the output pulse, reducing the thermal effect, and thus reducing the time width of the final femtosecond laser pulse and improving the shape of the pulse beam.
More specifically, the anisotropic gain material has different optical and thermal characteristics according to the axis of the material.
Accordingly, depending on the axis of the anisotropic gain material for the pumping light source to be applied and for the laser to be oscillated, the spectrum or the thermal characteristics of the output pulse may significantly vary.
However, while the two cited patents are considering only the improving effect of the output power as the number of laser materials increases without mentioning the axis of the laser material, the present invention contains the technical spirit that can enlarge the spectrum width or offset the thermal effect by selecting different axes of the laser material or pumping differently. Accordingly, the present invention is proposing that not only the laser output power can be enhanced, but also the pulse width can be decreased or the quality of the beam can be considerably increased.
On the other hand, a study on efficient absorption of a depolarized pumping light source and adjustment of the polarization rate of the pumping light source that pumps laser materials by selecting the axis of Yb:KVW laser material is disclosed in U.S. Pat. No. 6,891,876 B2.
The cited patent focuses mainly on optical pumping of an anisotropic material. First, the cited patent is proposing a method of effectively performing optical pumping on the laser materials by selecting the axis of the anisotropic gain material and the wavelength of the pumping laser even when the depolarized pumping light source is used.
Second, the cited patent is proposing a method of giving no change to the laser output by appropriately selecting the axis of the laser materials even in the case where the wavelength of the pumping light source is unstable and adjusting the polarization rate of incident pumping laser.
The proposed method shows that when the direction of the laser materials is determined and cut so that two axes can be appropriately combined in one laser material by using the fact that the absorption spectrum is different according to the axial direction of the laser material, and the intensity of the pumping light source is adjusted according to the polarization direction, similar absorption cross-sections can be obtained in a wide wavelength range.
However, the above is advantageous in that an optical pumping part less sensitive to the instability of the polarization or wavelength of the pumping light source can be manufactured, but in order to obtain the absorption basal that is similar in the large wavelength range, optical pumping needs to be performed in the wavelength range in which the absorption cross-section of the laser material is small.
Also, since the output needs to be divided into two polarized light with one laser light source, the pumping efficiency significantly decreases, and the pumping light source that is absorbed without being converted into the laser wavelength accumulate in the laser material as thermal energy, degrading the quality of the laser beam and restricting its output.
As described above, the laser pulse energy coming from the femtosecond oscillator generally falls within several nJ range, and is too low to be applied to the femtosecond laser processing, thereby requiring an amplification process for increasing the pulse energy.
However, since the laser material has a gain profile with a basically limited width, the amplification rate differs according to the wavelength of the pulse input and the gain narrowing in which the spectrum bandwidth of the amplified pulse is narrowed occurs, causing increase of the pulse time width.
More specifically, FIG. 1 is a graph illustrating a variation of the spectrum according to the gain narrowing.
If the input pulse of spectrum like (a) of FIG. 1 is applied to the laser materials that has a gain profile of a limited width like (b), the amplification is continuously performed at the central wavelength λc, but at the edge portion of wavelength that is away from the center, the gain is low and thus the amplification rate is smaller than that at the central wavelength.
When the laser pulse reciprocates in an amplifier resonator, the frequency of passing through the laser gain material increases, and a difference of an amplification ratio accumulates. Thus, the intensity at the edge wavelength becomes lower than that at the center wavelength.
In other words, it can be seen in a re-normalized graph of FIG. 1 that the width of the output pulse spectrum (c) is narrowed compared to the input pulse spectrum (a).
The dual tungstate laser material of the monoclinic system doped with ytterbium such as Yb:KYW and Yb:KGW has a wide emission bandwidth to the extent that the pulse of 100 fs or less can be made in the oscillator due to a large emission/absorption cross-section and large radiation bandwidth and the pulse of 200 fs or under can be generated in the amplifier.
However, due to the gain narrowing that occurs in the amplification process, the time width of the amplified femtosecond laser pulse stays at 300 fs to 400 fs.
Also, since the laser crystal of the Yb:KYW or Yb:KGW has excellent thermal conductivity, there is an advantage of manufacturing a femtosecond laser of a high average output power. However, since the anisotropic laser materials of Yb:KYW or Yb:KGW have different thermal conductivities, when the average output of the laser increases, the astigmatism of the thermal lenses may occur due to the thermal effect. Accordingly, if the shape of the laser beam is distorted, then the quality of the beam may be reduced.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.